Both monsters are destroyed, unless their ATK is 0, in which case neither is.

Neither monster is destroyed.

Attacking monster's ATK > Attack target's ATK

The controller of the attack target takes damage equal to the difference between the two monsters' ATK.

Neither player takes damage, unless the attacking monster inflicts piercing damage, in which case the controller of the attack target takes damage equal to the difference between the attacking monster's ATK and the attack target's DEF.